The invention relates to a memory constructed from a plurality of individual memory cells each comprising at least one field effect transistor having a memory gate which is enclosed all round by an insulating layer and is electrically floating. The transistor also has a further insulated gate, in particular an insulated, controllable control gate, wherein the erasure of the field effect transistor, i.e. the discharge of the floating gate, is carried out by means of a direct transfer of electrons from the floating gate through the insulating layer under the effect of a strong, connected electric field.
Such semiconductor memories are known whose individual memory cell contains at least one MOS field effect transistor which, in addition to a normal gate electrode, contains a so-called floating gate, an electrode which is capacitively coupled to the controllable gate of the transistor and which possesses neither an outer terminal nor a connection to the control gate electrode. The floating gate is separated from a conductive zone of the semiconductor body which is to be supplied by outer terminals only by an oxide layer which is so thin that when sufficiently "high" voltages are connected, a transfer of charge to and from the floating gate can occur. The charge stored in the floating gate represents the information which is stored in the cell and which experiences no change under normal operating voltages such as are used to read-out the memory. On the other hand, the "high" voltages are used for programming and for erasure. The term "high" voltage indicates that the voltage should be selected to be such that the exchange of charge between the floating gate and semiconductor body can take place without destroying the oxide layer lying therebetween, and in fact by a tunnel effect. There are many possible ways of effecting this, namely by using the so-called Fowler-Nordheim field emission.
U.S. Pat. No. 4,087,795 discloses an electronic memory having a plurality of electrically programmable storage field effect transistors which each possess an insulated, floating storage gate and a controllable control gate, wherein the erasure of the memory and the discharge of the floating gate are carried out by electrical means, and in fact by means of an erasing voltage supplied between the control gate and the channel path. The programming of the memory, i.e. the charging of the floating storage gate is carried out on the other hand at a different position in the field effect transistor and also with a different physical mechanism, namely by means of channel injection. For this purpose charge carriers are accelerated in a short channel and the so-called hot charge carriers are conveyed to the storage gate with the aid of an additional electrical transverse field.
With this method of recording and erasing in memories, i.e. the charging and discharging of floating storage gates by means of different physical mechanisms, there are a series of technical difficulties which lead to disadvantages in the memory produced by the method.
First the insulating layer thickness in the erasing zone of these memory cells cannot be arbitrarily selected. In order to simplify the dimensioning of the peripheral electronics and the voltage supply and simultaneously to reduce the power loss, on the one hand it would be desirable to reduce the erasing voltage by reducing the insulating layer thickness in the erasing zone. On the other hand, on account of the danger of the disturbance of neighboring words, it should be ensured that the erasing voltage does not come into the order of the programming voltages (e.g. U.sub.g =15 V). In this case, for example, when a "1" is written in by means of channel injection into a selected memory cell, a "1" would also be entered in those memory cells which, although they have no channel current flowing therein, exhibit a sufficiently high voltage between their floating gates and associated channel zones so that the corresponding floating gates are charged by means of the Fowler-Nordheim effect through the thin insulating layer in the erasing zone. Misprogramming of this type is also promoted by the technologically dependent tolerance fluctuations in the insulating layer thicknesses.
Secondly, programming with hot charge carriers by avalanche injection or channel injection always requires a current between the two diffusion zones and between a diffusion zone and the substrate. This current is considerably greater than the actual recharging current of the floating storage gate and gives rise to additional problems during word-wise programming of e.g. 8 to 16 bits. The resulting disadvantages consist, for example, of a shorter life duration, higher power losses, higher current requirements on the mains components and an increase in the dimensions of the peripheral electronics.
Furthermore when hot charge carriers are used to record or to erase, the danger of walkout problems always exists which increase the erasing and progamming duration of such memories and reduce the life duration.